Electrostatic discharge protection of thin-film resonators

ABSTRACT

A filter having a thin-film resonator fabricated on a semiconductor substrate and a method of making the same are disclosed. The filter has a bonding pad connected to the resonator and in contact with the substrate to form a Schottky diode with the substrate to protect the resonator from electrostatic discharges.

This is a Divisional of application Ser. No. 10/209,602, filed on Jul.30, 2002, now U.S. Pat. No. 6,894,360 the entire disclosure of which isincorporated herein by reference.

BACKGROUND

The present invention relates to acoustic resonators, and moreparticularly, to resonators that may be used as filters for electroniccircuits.

The need to reduce the cost and size of electronic equipment has led toa continuing need for ever-smaller electronic filter elements. Consumerelectronics such as cellular telephones and miniature radios placesevere limitations on both the size and cost of the components containedtherein. Further, many such devices utilize electronic filters that mustbe tuned to precise frequencies. Filters select those frequencycomponents of electrical signals that lie within a desired frequencyrange to pass while eliminating or attenuating those frequencycomponents that lie outside the desired frequency range.

One class of electronic filters that has the potential for meeting theseneeds is constructed from thin film bulk acoustic resonators (FBARs).These devices use bulk longitudinal acoustic waves in thin filmpiezoelectric (PZ) material. In one simple configuration, a layer of PZmaterial is sandwiched between two metal electrodes. The sandwichstructure is preferably suspended in air. A sample configuration of anapparatus 10 having a resonator 12 (for example, an FBAR) is illustratedin FIGS. 1A and 1B. FIG. 1A illustrates a top view of the apparatus 10while FIG. 1B illustrates a side view of the apparatus 10 along line A—Aof FIG. 1A. The resonator 12 is fabricated above a substrate 14.Deposited and etched on the substrate 14 are, in order, a bottomelectrode layer 15, piezoelectric layer 17, and a top electrode layer19. Portions (as indicated by brackets 12) of these layers—15, 17, and19—that overlap and are fabricated over a cavity 22 constitute theresonator 12. These portions are referred to as a bottom electrode 16,piezoelectric portion 18, and a top electrode 20. In the resonator 12,the bottom electrode 16 and the top electrode 20 sandwiches the PZportion 18. The electrodes 14 and 20 are conductors while the PZ portion18 is typically crystal such as Aluminum Nitride (AlN).

When electric field is applied between the metal electrodes 16 and 20,the PZ portion 18 converts some of the electrical energy into mechanicalenergy in the form of mechanical waves. The mechanical waves propagatein the same direction as the electric field and reflect off of theelectrode/air interface.

At a resonant frequency, the resonator 12 acts as an electronicresonator. The resonant frequency is the frequency for which the halfwavelength of the mechanical waves propagating in the device isdetermined by many factors including the total thickness of theresonator 12 for a given phase velocity of the mechanical wave in thematerial. Since the velocity of the mechanical wave is four orders ofmagnitude smaller than the velocity of light, the resulting resonatorcan be quite compact. Resonators for applications in the GHz range maybe constructed with physical dimensions on the order of less than 100microns in lateral extent and a few microns in total thickness. Inimplementation, for example, the resonator 12 is fabricated using knownsemiconductor fabrication processes and is combined with electroniccomponents and other resonators to form electronic filters forelectrical signals.

The use and the fabrication technologies for various designs of FBARsfor electronic filters are known in the art and a number of patents havebeen granted. For example, U.S. Pat. No. 6,262,637 granted to Paul D.Bradley et al. discloses a duplexer incorporating thin-film bulkacoustic resonators (FBARs). Various methods for fabricating FBARs alsohave been patented, for example, U.S. Pat. No. 6,060,181 granted toRichard C. Ruby et al. discloses various structures and methods offabricating resonators, and U.S. Pat. No. 6,239,536 granted to KennethM. Lakin discloses method for fabricating enclosed thin-film resonators.

However, the continuing drive to increase the quality and reliability ofthe FBARs presents challenges requiring even better resonator quality,designs, and methods of fabrication. For example, one such challenge isto eliminate or alleviate susceptibility of the FBARs from damages fromelectrostatic discharges and voltage spikes from surrounding circuits.Another challenge is to eliminate or alleviate susceptibility of theresonator from frequency drifts due to interaction with its environmentsuch as air or moisture.

SUMMARY

These and other technological challenges are met by the presentinvention. According to one aspect of the present invention, anelectronic filter includes a thin-film resonator fabricated on asemiconductor substrate and bonding pad connected to the thin-filmresonator, the bonding pad forming a Schottky diode with the substrateto protect the thin-film resonator from electrostatic discharges.

According to another aspect of the present invention, a method forfabricating an electronic filter is disclosed. A thin-film resonator isfabricated on a substrate. With the resonator, a bonding pad isfabricated, the bonding pad being connected to the thin-film resonatorand a portion of the bonding-pad being in contact with the substrate toform a Schottky diode.

Other aspects and advantages of the present invention will becomeapparent from the following detailed description, taken in combinationwith the accompanying drawings, illustrating by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a top view of an apparatus including a resonator known inprior art;

FIG. 1B is a side view of the apparatus of FIG. 1A cut along line A—A;

FIG. 2A is a top view of an apparatus according to a first embodiment ofthe present invention;

FIG. 2B is a side view of the apparatus of FIG. 2A cut along line B—B;

FIG. 3A is a top view of an apparatus according to a second embodimentof the present invention;

FIG. 3B is a side view of the apparatus of FIG. 3A cut along line C—C;

FIG. 4A is a top view of an apparatus according to a third embodiment ofthe present invention;

FIG. 4B is a side view of the apparatus of FIG. 4A cut along line D—D;and

FIG. 4C is a schematic diagram illustrating, in part, a circuit that canbe formed using the apparatus of FIG. 4A.

DETAILED DESCRIPTION

As shown in the drawings for purposes of illustration, the presentinvention is embodied in a filter having a thin-film resonatorfabricated on a semiconductor substrate. Bonding pad connecting theresonator to the rest of the filter circuit is connected to theresonator. The bonding pad is in contact with the substrate therebyforming a Schottky junction diode. In normal operations, the diode is anopen circuit and does not affect the operations of the filter. When anelectrostatic discharge (ESD) spike voltage is introduced to theresonator via the bonding pad, the diode closes thereby discharging theESD voltage to the substrate thereby protecting the resonator.

FIG. 2A illustrates a top view of an apparatus 30 according to a firstembodiment of the present invention. FIG. 2B is a side view of theapparatus 30 of FIG. 2A cut along line B—B. Portions of the apparatus 30in FIGS. 2A and 2B are similar to those of the apparatus 10 of FIGS. 1Aand 1B. For convenience, portions of the apparatus 30 in FIGS. 2A and 2Bthat are similar to portions of the apparatus 10 of FIGS. 1A and 1B areassigned the same reference numerals and different portions are assigneddifferent reference numerals. Referring to FIGS. 2A and 2B, theapparatus 30 according to one embodiment of the present inventionincludes a resonator 32 fabricated on a substrate 14. The apparatus 30is fabricated first be etching a cavity 34 into the substrate 14 andfilling it with suitable sacrificial material such as, for example,phosphosilicate glass (PSG). Then, the substrate 14, now including thefilled cavity 34 is planarized using known methods such as chemicalmechanical polishing. The cavity 34 can include an evacuation tunnelportion 34 a aligned with an evacuation via 35 through which thesacrificial material is later evacuated.

Next, a thin seed layer 38 is fabricated on the planarized substrate 14.Typically the seed layer 38 is sputtered on the planarized substrate 14.The seed layer 38 can be fabricated using Aluminum Nitride (AlN) orother similar crystalline material, for example, Aluminum Oxynitride(ALON), Silicon Dioxide (SiO₂), Silicon Nitride (Si₃N₄), or SiliconCarbide (SiC). In the illustrated embodiment, the seed layer 38 is inthe range of about 10 Angstroms (or one nanometer) to 10,000 Angstroms(or one micron) thick. Techniques and the processes of fabricating aseed layer are known in the art. For example, the widely known and usedsputtering technique can be used for this purpose.

Then, above the seed layer 38, the following layers are deposited, inorder: a bottom electrode layer 15, a piezoelectric layer 17, and a topelectrode layer 19. Portions (as indicated by brackets 32) of theselayers—36, 15, 17, and 19—that overlap and are situated above the cavity34 constitute the resonator 32. These portions are referred to as a seedlayer portion 40, bottom electrode 16, piezoelectric portion 18, and topelectrode 20. The bottom electrode 16 and the top electrode 20sandwiches the PZ portion 18.

The electrodes 14 and 20 are conductors such as Molybdenum and, in asample embodiment, are in a range of 0.3 micron to 0.5 micron thick. ThePZ portion 18 is typically made from crystal such as Aluminum Nitride(AlN), and, in the sample embodiment, is in a range from 0.5 micron to1.0 micron thick. From the top view of the resonator 32 in FIG. 2A, theresonator can be about 150 microns wide by 100 microns long. Of course,these measurements can vary widely depending on a number of factors suchas, without limitation, the desired resonant frequency, materials used,the fabrication process used, etc. The illustrated resonator 32 havingthese measurements can be useful in filters in the neighborhood of 1.92GHz. Of course, the present invention is not limited to these sizes orfrequency ranges.

The fabrication of the seed layer 38 provides for a better underlayer onwhich the PZ layer 17 can be fabricated. Accordingly, with the seedlayer 38, a higher quality PZ layer 17 can be fabricated, thus leadingto a higher quality resonator 32. In fact, in the present sampleembodiment, the material used for the seed layer 38 and the PZ layer 17are the same material, AlN. This is because seed layer 38 nucleates asmoother, more uniform bottom electrode layer 15 which, in turn,promotes a more nearly single crystal quality material for the PZ layer17. Thus, piezoelectric coupling constant of the PZ layer 17 isimproved. The improved piezoelectric coupling constant allows for widerbandwidth electrical filters to be built with the resonator 32 and alsoyields more reproducible results since it tightly approaches thetheoretical maximum value for AlN material.

FIG. 3A illustrates a top view of an apparatus 50 according to a secondembodiment of the present invention. FIG. 3B is a side view of theapparatus 50 of FIG. 3A cut along line C—C. Portions of the apparatus 50in FIGS. 3A and 3B are similar to those of the apparatus 30 of FIGS. 2Aand 2B. For convenience, portions of the apparatus 50 in FIGS. 3A and 3Bthat are similar to portions of the apparatus 30 of FIGS. 2A and 2B areassigned the same reference numerals and different portions are assigneddifferent reference numerals.

Referring to FIGS. 3A and 3B, the apparatus 50 of the present inventionincludes a resonator 52 fabricated on a substrate 14. The apparatus 50is fabricated similarly to the apparatus 30 of FIGS. 2A and 2B anddiscussed herein above. That is, bottom electrode layer 15,piezoelectric layer 17, and top electrode layer 19 are fabricated abovea substrate 14 having a cavity 34. Optionally, a seed layer 38 isfabricated between the substrate 14 including the cavity 34 and thebottom electrode layer 15. Details of these layers are discussed above.The resonator 52 comprises portions (as indicated by brackets 52) ofthese layers—36, 15, 17, and 19—that overlap and are situated above thecavity 34. These portions are referred to as a seed layer portion 40,bottom electrode 16, piezoelectric portion 18, and top electrode 20.Finally, a protective layer 54 is fabricated immediately above the topelectrode 20. The protective layer 54 covers, at least, the topelectrode 20, and can cover, as illustrated, a larger area than the topelectrode 20. Moreover, portion of the protective layer 54 that issituated above the cavity 34 is also a part of the resonator 52. Thatis, that portion of the protective layer 54 contributes mass to theresonator 52 and resonates with all the other parts—40, 16, 18, and20—of the resonator 52.

The protective layer 54 chemically stabilizes and reduces the tendencyof material to adsorb on the surface of the top electrode 20. Adsorbedmaterial can change the resonant frequency of the resonator 32. Thethickness may also be adjusted to optimize the electrical quality factor(q) of the resonator 32.

Without the protective layer 54, resonant frequency of the resonator 52is relatively more susceptible to drifting over time. This is becausethe top electrode 20, a conductive metal, can oxidize from exposure toair and potentially moisture. The oxidization of the top electrode 20changes the mass of the top electrode 20 thereby changing the resonantfrequency. To reduce or minimize the resonant frequency-driftingproblem, the protective layer 54 is typically fabricated using inertmaterial less prone to reaction with the environment such as AluminumOxynitride (ALON), Silicon Dioxide (SiO₂), Silicon Nitride (Si₃N₄), orSilicon Carbide (SiC). In experiments, the protective layer 54 havingthickness ranging from 30 Angstroms to to 2 microns have beenfabricated. The protective layer 54 can include AlN material, which canalso be used for the piezoelectric layer 17.

Here, the seed layer portion 40 not only improves the crystallinequality of the resonator 52, but also serves as a protective underlayerprotecting the bottom electrode 16 from reaction with air and possiblemoisture from the environment reaching the bottom electrode 16 via theevacuation via 35.

FIG. 4A illustrates a top view of an apparatus 60 according to a thirdembodiment of the present invention. FIG. 4B is a side view of theapparatus 60 of FIG. 4A cut along line D—D. FIG. 4C is a simpleschematic illustrating, in part, an equivalent circuit that can beformed using the apparatus 60. Portions of the apparatus 60 in FIGS. 4A,4B, and 4C are similar to those of the apparatus 10 of FIGS. 1A and 1Band the apparatus 30 of FIGS. 2A and 2B. For convenience, portions ofthe apparatus 60 in FIGS. 4A, 4B, and 4C that are similar to portions ofthe apparatus 10 of FIGS. 1A and 1B and portions of the apparatus 30 ofFIGS. 2A and 2B are assigned the same reference numerals and differentportions are assigned different reference numerals.

Referring to FIGS. 4A, 4B, and 4C, the apparatus 60 is fabricatedsimilarly to the apparatus 10 of FIGS. 1A and 1B and discussed hereinabove. That is, bottom electrode layer 15, piezoelectric layer 17, andtop electrode layer 19 are fabricated above a substrate 14 having acavity 22. These layers are fabricated in a similar manner as theapparatus 30 of FIGS. 2A and 2B and the details of these layers arediscussed above. The resonator 12, preferably a thin-film resonator suchas an FBAR, comprises portions (as indicated by brackets 12) of theselayers—15, 17, and 19—that overlap and are situated above the cavity 22.These portions are referred to as bottom electrode 16, piezoelectricportion 18, and top electrode 20.

The apparatus 60 includes at least one bonding pad. Illustrated in FIGS.4A and 4B are a first bonding pad 62 and a second bonding pad 64. Thefirst bonding pad 62 is connected to the resonator 12 by its topelectrode layer 19. The first boding pad 62 is in contact with thesemiconductor substrate 14 thereby forming a Schottky junction diode 63.Operational characteristics of such diodes are known in the art.

Also illustrated is a second bonding pad 64 connected to the resonator12 by its bottom electrode layer 15. The second bonding pad 64 isillustrated as making contact with the substrate 14 at two placesthereby forming two Schottky diode contacts 65. In fact, a bonding padcan be fabricated to form, in combination with the substrate 14, aplurality of diode contacts for the protection of the resonator to whichit is connected. The contacts 65 from a single pad 64 form,electrically, a single Schottky diode.

The bonding pads 62, 64 are typically fabricated using conductive metalsuch as gold, nickel, chrome, other suitable materials, or anycombination of these.

FIG. 4C can be used to used to describe the operations of the filtercircuit 72 having the resonator 12. Normally, no current flows throughthe diodes 63 and 65 as the diode 63 operate as an open circuit in onedirection while diode 65 operates as a closed circuit in the oppositedirection. However, when an electrostatic voltage spike is introduced tothe resonator 12 via its bonding pad 64 (from, perhaps, an antennae 66),the diode 63 breaks down. When the diode 63 breaks down, it iseffectively a closed short circuit, and allows the voltage spike to betransferred to the substrate 14, and eventually ground 68, therebyprotecting the resonator 12 from the voltage spike. The other diode 65operates similarly to protect the resonator 12 from voltage spikes fromother electronic circuits 70 connected to the filter 72. That is, twometal pads, for example pads 62 and 64 connected to electricallyopposing sides of the resonator 12, fabricated on semiconductorsubstrate create an electrical circuit of two back-to-back Schottkydiodes which allow high voltage electrostatic discharges to dissipateharmlessly in the substrate rather than irreversibly breaking down thepiezoelectric layer, for example PZ layer 17, which separates top andbottom electrodes, for example electrodes 16 and 20, from each other. Anelectronic schematic diagram of FIG. 4C illustrates such connection.

In an alternative embodiment, a single apparatus can include a resonatorhaving all of the features discussed above including the seed layer 38and the protective layer 54 illustrated in FIGS. 2A, 2B, 3A and 3B andbonding pads 62 and 64 (forming Shottkey diodes 63 and 65) illustratedin FIGS. 4A and 4B. In the alternative embodiment, the pads 62 and 64can be formed on the seed layer 38 with several microns of overhang overand beyond the top electrode layer 19 and the bottom electrode layer 15.

From the foregoing, it will be appreciated that the present invention isnovel and offers advantages over the current art. Although a specificembodiment of the invention is described and illustrated above, theinvention is not to be limited to the specific forms or arrangements ofparts so described and illustrated. The invention is limited by theclaims that follow.

1. A method of fabricating an apparatus, the method comprising:fabricating a bottom electrode layer on a substrate, the bottomelectrode layer having an opening; fabricating a piezoelectric layer onthe bottom electrode layer and on the substrate; fabricating a topelectrode layer on the piezoelectric layer and on the substrate; whereinoverlapping portions of the bottom electrode layer, the piezoelectriclayer, and the top electrode layer forming a resonator; and fabricatinga bonding pad on the bottom electrode layer, the bonding pad in contactwith the substrate through the opening of the bottom electrode, thebonding pad and the substrate forming a diode.
 2. The method recited inclaim 1 wherein the top electrode layer has an opening; and furthercomprising a step of fabricating a bonding pad on the top electrodelayer, the bonding pad in contact with the substrate through the openingof the top electrode, the bonding pad and the substrate forming a diode.3. The method recited in claim 1 further comprising a step offabricating a seed layer under the bottom electrode layer.
 4. The methodrecited in claim 1 wherein the seed layer having an opening aligned withthe opening of the bottom electrode allowing the bonding pad to contactthe substrate through the opening.
 5. The method recited in claim 1wherein said bonding pad forms a Schottky diode with the substrate. 6.The method recited in claim 1 wherein said bonding pad comprisesconductor selected from a group consisting of gold, nickel, and chrome.7. The method recited in claim 1 wherein the piezoelectric layercomprises Aluminum Nitride and said bottom and top electrode layerscomprises Molybdenum.
 8. The method recited in claim 1 wherein thebottom electrode includes a plurality of openings through which thebonding pad contacts the substrate.